A ring laser angular rotation sensor is frequently called a ring laser angular rate sensor or a ring laser gyro. It will be called a ring laser angular rotation sensor herein. It uses a ring laser which is usually, but not necessarily, within a solid block of, for example, quartz or ceramic material. That block, or the equivalent supporting structure for the laser path, will be called and defined herein as the ring laser body.
It must here be noted that early ring laser rotation sensors did not use a solid block for the ring laser but used either a single linear laser in one of the legs or a plurality of linear lasers in the different legs to produce the laser beam. The laser or lasers were attached to a supporting structure along with corner mirrors to complete the ring of light. It is within the contemplation of this invention that ring laser apparatus of that type be included in the concept of a "ring laser body." The ring laser will further be described as if the ring laser path were completely enclosed within a solid block.
A laser body may, for example, be triangular or rectangular, and hollow bores are drilled or otherwise formed within or on the laser body. Mirrors are positioned to cause light to travel from mirror to mirror around a closed path through the bores. The bores typically enclose a lasing gas such as, for example, a helium-neon mixture. To cause the ring laser to become a ring laser angular rotation sensor, means are provided to cause two counter-propagating coherent laser beams to be produced and reflected around the closed path. Partly transparent mirrors are typically used to extract from the ring laser path a portion of the two counter-propagating beams. The two extracted beams are typically directed onto an electrical photo-sensor which is low frequency limited to produce a signal which has a frequency equal to the difference frequency between the two counter-propagating beams. The ring laser angular rotation sensor has a sensor axis inside of the closed ring laser path. When the laser angular rotation sensor is not rotating about its sensor axis, the laser light frequencies of the two counter-propagating laser beams are the same. When the laser angular rotation sensor is rotating about its sensor axis, the frequency of one beam increases and the frequency of the other beam decreases. The difference in frequency between the two counter-propagating beams is a measure of the angular velocity of the angular rotation sensor about its sensor axis. Counting the beats between the two counter-propagating beams provides a measure of the angular displacement of the angular rotation sensor about its sensor axis.
Because of scattering at the mirror surfaces and other factors, the frequencies of the counter-propagating laser beams lock together when the angular velocity of the angular rotation sensor about its sensor axis has a value that is below a particular threshhold. This phenomenon is called, "lock-in". One preferred method of avoiding or minimizing the effects of lock-in is mechanically to oscillate or dither the ring laser angular rotation sensor about its sensor axis.
Still another means for applying an oscillatory bias to the ring laser beams is described in the literature, particularly U.S. Pat. No. 3,373,650 which issued Mar. 19, 1968 to J. E. Killpatrick. In U.S. Pat. No. 3,373,650, a Faraday cell and two quarter-wave plates are inserted into the ring laser light path. The Faraday cell is enclosed in a coil which is energized by an oscillatory current to produce an oscillatory magnetic field that via the Faraday cells changes the phase of each laser beam a different amount thereby biasing the two counterpropagating laser beams with an oscillatory bias.
Although the invention was conceived in connection with laser gyros having mechanical bias, it is intended that the concept, in its broadly claimed form, should encompass the Faraday cell dithered bias concept as well as the mechanically dithered bias concept.
In the mechanically dithered bias, the bias is delivered to the laser body by the spring, and the mechanical inertia of the body causes the body to oscillate on the spring.
In the Faraday cell dithered bias, the bias is delivered to the laser beam by the magnetic field, and the magnetic field inertia or resistance to change is equivalent to the inertia of the mechanical body.
The driving function for the mechanical embodiment is the torque applied to the body. Such torque may be applied directly between the body and the supporting structure, or it may be delivered through the supporting springs. Such torque may change quickly, but the angular velocity of the laser body changes more slowly due to the inertia of the body and the spring constant of the spring.
The driving function for the Faraday cell embodiment is the voltage applied to the Faraday cell coil. Such voltage may change quickly, but the magnetic field and coil current change more slowly due to the magnetic resistance-inductance time constants of the circuit.
The invention will be described in its mechanical embodiment, but the equivalent Faraday cell structure will occasionally be mentioned.
As described generally in U.S. Pat. No. 3,373,650, both apparatus are a device wherein two beams of monochromatic light are generated along a closed loop path in two opposite directions and the frequency difference between the two beams is determined as a measure of rotation thereof. The apparatus to prevent the lock-in of the two beams of light comprises a means of biasing the beams of light at different frequencies; and means causing the bias periodically to reverse.
To minimize the amount of energy required to oscillate or dither the ring laser angular rotation sensor, it is preferable to mount the angular rotation sensor body upon a spring structure and to dither or oscillate the body and spring structure about the sensor axis at the natural frequency of the angular rotation sensor mass and the spring structure. The amplitudes of the oscillation or dithering are carefully controlled and monitored to minimize the lock-in effects. Because the dither oscillation angular velocity and displacement relative to a support structure can constantly be monitored and measured, they may be excluded from the output signal of the angular rotation sensor to produce signals that are measures of the angular velocity and displacement of the angular rotation sensor-support structure about the sensor axis of the angular rotation sensor.
It has been found that a constant amplitude dithering of the laser angular rotation sensor about its sensor axis is inadequate totally to eliminate lock-in error. It was first suggested that the superimposing of a random signal upon the amplitude of the dither driving amplifier would be satisfactory. However, it was found that a substantial error was produced by the use of such random signal.
One structure for mounting angular rotation sensors is to mount them upon a gimballed system. Typically the sensing axes of the angular rotation sensors are held fixed relative to inertial space or relative to ground coordinates.
Still another mechanization is to attach the angular rotation sensors to the vehicle so that the sensor axes of the angular rotation sensors are aligned with a set of orthogonal axes upon the aircraft. So, too, accelerometers are positioned on the vehicle. A computer continuously transforms the information in vehicle coordinates into desired navigation coordinates. Such a mechanism is called a strapped down mechanization. In a strapped down mechanization, it is not unusual to have shock mounts between the vehicle and the instruments.
Because of the scale factor accuracy and input range of the ring laser angular rotation sensor, it is much better suited for a strapped down system than a spinning rotor gyro. The scale factor accuracy advantage of a ring laser rotation sensor is typically five to ten times more accurate than a spinning rotor gyro.
When more than one ring laser angular rotation sensor is strapped down, it is customary to support the angular rotation sensors upon a platform or mounting structure which is supported by low-pass shock mounts relative to the vehicle. If the natural frequencies of the mechanical mass-spring combinations of more than one ring laser angular rotation sensor connected to the same platform or mounting structure are the same, the oscillation of one mass-spring combination may excite oscillation in another mass-spring combination. The mechanical interaction between the mass-spring combinations of the angular rotation sensors (typically three) on the platform or mounting structure produces complex angular motions which are functions of the sums and differences of each of the angular rotation sensor dither frequencies. If the frequencies of two or more angular rotation sensors are the same or near the other, coning or Scorsby motion can occur about an axis or about all axes of the platform or mounting structure. To minimize such exciting or coupling between the mass-spring combinations, it is customary to select the mass-spring combinations of the angular rotation sensors to have different natural frequencies. Although they may be larger, usually the differences in natural frequencies of the mass-spring combinations of the laser angular rotation sensors are on the order of five to ten hertz. A typical three decibel band width of a ring laser mass-spring system is on the order of five hertz.
In a mechanically dithered system, coning motion of the supporting platform or mounting structure arises because of reaction torque through the support or platfrom from one mass-spring system to itself or another. The input coning motion produces angular rotation sensor-sensed coning rate. Although coning motion is normally larger when a mass-spring system has a natural frequency which is within the three decibel bandwidth of the natural frequency of another mass-spring system, unacceptable coning errors may exist when the natural frequencies differ by several bandwidths. Note that the coning motions may be induced about any or all axes regardless of the ring laser angular rotation sensor mutual mounting orientations.
In a Faraday cell dithered system, coning motion of the supporting platform or mounting structure arises because of magnetic coupling between the Faraday cells in the various ring lasers. It would, in the prior art, be desirable to separate the dithering frequencies of the plurality (usually three) ring laser rotation sensors so that the bandwidths of the sensors do not overlap.
The following example is preliminary to further explaining coning errors. Suppose a right handed rectangular coordinate system is positioned at the zero meridian on the equator of the earth with its x axis pointing east, its y axis pointing north along the zero meridian, and its z axis vertical.
In the following described motions the z axis is maintained vertical, and the angular rate of the x and y coordinates about the z axis is held to zero.
The coordinate system is moved eastward ninety degrees of longitude. The x axis still points east and the y axis points north along the ninety degree east meridian.
The coordinate system is then moved north ninety degrees of latitude. The x and y axes now both point south. The x axis points along the one hundred and eighty degrees meridian. The y axis points along the 90 degree west meridian.
The coordinate system is then moved south along the zero meridian to its starting point. The x axis then points north along the zero meridian, and the y axis points west.
The apparent change in direction is called the wander angle. Note that the change in orientation is ninety degrees. That is not a coincidence. For any closed vehicle trajectory on a sphere the wander azimuth change equals the ratio of the area enclosed by the trajectory to the area of the sphere. In particular, if an aircraft continually circles in a holding pattern, the wander azimuth angle changes at a constant rate determined by the area enclosed by the holding pattern and the rate at which the pattern trajectories are completed.
The discussion is directed to a system wherein three angular rotation sensors are positioned on a platform or mounting structure with each angular rotation sensor measuring angular velocity about one of the orthogonal coordinate axes, but the discussion is equally valid in a strapped down system wherein the angular velocity coordinates are computed and stored in a computer memory.
Consider a strapped down system. The x,y and z axes are angularly constrained relative to a vehicle. The constraint may be through low pass shock mounts. Further, because of the resiliency of the angular rotation sensor platform or mounting structure the angular rotation sensors may be considered to be attached together by very stiff springs. The dither oscillations of each of the three angular rotation sensors mounted upon the platform delivers some portion of that dither oscillation to the platform thence to the other angular rotation sensors. In the mechanical embodiment, the coupling is mechanical and through the support structure. In the Faraday cell embodiment, the coupling is magnetic.
In an example it is assumed that the angular motions about both the x and y axes are of equal amplitude and sinusoidal, but the sinusoids are ninety degrees out of phase. The x and y axes then each trace out a figure eight motion. The z axis traces out a circle of radius r where r is the amplitude, in radians, of the sinusoids delivered about the x and y axes.
In a Faraday cell embodiment, the axes orientations would be slightly different, but the principle would be the same.
For a numerical example, assume:
The amplitude of the oscillations about the pitch and roll axes are plus or minus 0.01 radian (0.573 degrees). Then the area of the circle is EQU dA=(3.14159)(0.01)(0.01)=0.000314159
If the period of the oscillation, dt, is 0.0025 seconds will indicate that the sensed angular rate about such z axis is EQU dA/dt=0.125836 radians or 6.68 degrees per second
even though the average azimuth change is zero.
The above example illustrates pure "coning" in which a body undergoes sinusoidal vibrations about orthogonal axes, and in which the vibrations are ninety degrees out of phase so that a fixed body z axis traces out a cone. For the z axis coning rate to exist, the motions delivered to the x and y axes need not be sinusoidal and the time relation between such signals need not be constant. As in the example, it is only necessary that some axis (referred to here as "z") trace out a closed path on a unit sphere. If the phase relations between the different axes are random, positive areas cancel negative areas as in a "random walk", but the angular change will still build up with the square root of time. Note that even if the motion were random, the phase angles would be correlated out of phase causing an average drift in one direction.
In a Faraday cell embodiment, the above example would correspond to the situation wherein the three magnetic fields would be coupled, and sinusoidal vibrations of the magnetic fields are out of phase and couple into the Faraday cell of another ring laser rotation sensor whose sensing axis is differently oriented on the supporting structure (usually orthogonal).